Hydronic design ties flow, load and temperature drop together: Btu per hour equals about 500 times gpm times delta T for water. Classic systems were designed for a 20 degree F delta T; condensing systems often use 30 to 40 to keep return water cold. Measuring delta T across a boiler or a zone tells a tech whether flow is too high, too low or about right.
Delta T across the boiler is 4 degrees, way too much flow, the condensing boiler's never going to condense like that.
Also heard as
- ΔT
- temperature drop
- TD
Related terms
Primary-secondary
Describes hydronic piping where each circuit has its own pump and connects to a main loop through two closely spaced tees.
Condensing boiler
A boiler designed to cool flue gas below its dew point so water vapor condenses and gives up its latent heat.
Outdoor reset
A control strategy that lowers hot water supply temperature as outdoor temperature rises, following a set curve.
Short cycling
A burner starting and stopping far too often, with short run times that waste fuel and wear parts.